We calculate the rho meson mass in a weak magnetic field using effective $rhopipi$ interaction. It is seen that both $rho^0$ and $rho^pm$ masses decrease with the magnetic field in vacuum. $rho$ meson dispersion relation has been calculated and shown to be different for $rho^0$ and $rho^pm$. We also calculate the $rhopipi$ decay width and spectral functions of $rho^0$ and $rho^pm$. The width is seen to decrease with $eB$ and the spectral functions become narrower.